The present invention relates to methods and apparatuses for heating liquid in an electric kettle.
Electric kettles have long been used to heat liquids such as water. Generally, electric kettles include a temperature sensor to measure the temperature of the liquid being heated. Many kettles include a time sensor as well.
It is often possible to determine when the liquid contained in the kettle has reached its boiling point by comparing temperature and time measurements. More specifically, when the temperature of the liquid being heated ceases to change with respect to time, it can be deduced that the liquid""s boiling point has been reached. This is because liquid generally cannot be heated to a temperature above its boiling point. Upon reaching the boiling point, a phase transition takes place and all energy is utilized to convert the liquid into gas rather than to heat the liquid. A method of determining when a liquid has reached its boiling point, similar to the method discussed above, was disclosed in European patent application EP 0 380 369 A1, which is incorporated by reference herein.
Many electric kettles have been configured to regulate temperature of the liquids they contain. For example, European patent application EP 0 704 186 A1, incorporated by reference herein, discloses a method for regulating the water temperature in an electric kettle. The water is heated from an initial temperature to a high temperature (less than the boiling point). The remaining heating time required before the water reaches its boiling point is then determined as a function of temperature and time. More particularly, based on the heating time required for the liquid to be heated from its initial temperature to its high temperature, the amount of time required to heat the water from its high temperature to its boiling point can be deduced. Therefore, electric kettles of this sort heat liquid for a calculated amount of time, and then shut off.
In order to prevent overheating of the kettle itself, many electric kettles include overheating protection devices. For example, many kettles include fusible cutouts, bimetal sensors, and/or other mechanical sensors. Other kettles prevent overheating of the kettle by monitoring and regulating temperature of the kettle""s heating element.
In European patent application EP 0 380 369 A1, a method is disclosed for detecting overheating of the kettle and preventing dry boiling, i.e., heating of an empty kettle. In order to detect and prevent overheating of the kettle, the temperature of the heating plate of the kettle is measured using a temperature sensor. An excessive increase in temperature of the heating plate is interpreted as a signal that the electric kettle contains too little liquid or no liquid at all. Upon reaching a temperature over a predetermined maximum limit, the kettle is shut off.
The apparatuses and methods discussed above occasionally fail due to defective or inaccurate sensors and/or poor positioning of sensors. Defective temperature sensors generally convey an erroneous temperature measurement. Even properly functioning temperature sensors frequently convey inaccurate temperature readings because they are positioned too far away from the source of heat. For example, according to EP 0 380 369 A1, a defined overtemperature must be reached before the electric kettle switches off. But, heating often takes place above this temperature because the respective temperature sensors are not arranged directly at the heating element. For this reason, an even higher temperature is present at the sensor, until the thermal gradient around the heating element has reached thermal equalization. Similarly, any safety switch-off by means of a fusible cutout only takes place if considerably more thermal energy is supplied to the system than would be supplied during operation at nominal rating.
As noted above, an erroneous temperature measurement may cause the functioning of the apparatuses to be altered earlier or later than desired. For example, upon detecting an erroneously low liquid temperature, the regulated kettle may be shut off too late causing the temperature of the liquid to exceed a desired temperature. Or, upon reading an erroneously low heat plate temperature, the kettle may be shut off too late causing the temperature of the kettle itself to exceed a desired maximum temperature limit. Similarly, erroneously high liquid temperature measurements or heat plate temperature measurements can cause the kettle to be shut off too early resulting in a liquid temperature that is less than the temperature desired.
According to one aspect of the invention, during heating of a liquid, thermal overshoot resulting from deactivating a heating element too late is prevented. Furthermore, a time delay between the point in time when the heating is switched on and when a temperature increase at a sensor can be detected is taken into consideration. In spite of different fill quantities and different values for a difference between a target temperature and a starting temperature, it is possible to reproducibly achieve a preselected target temperature.
The preset target temperature may be below boiling temperature. This is the case for example if water or mineral water which has been boiled before is used for preparing baby food. Another example includes the preparation of green tea with water or mineral water that has been boiled before.
Precisely when using water which has been boiled before, very slight differences between the target temperature and the starting temperature may occur. Furthermore, this means that the starting temperature can be either above or below the ambient temperature.
If the water fill level is low and, thus, the heat capacity of the system is low, and if the temperature sensor responds relatively slowly, i.e. the delay time is relatively long, the temperature difference between the preselected target temperature and the start temperature that was measured may be insufficient for the heating process to be carried out at full heating output without overshoot occurring. In this case, the temperature difference is the minimum acceptable temperature difference or the reference temperature difference. In this case, a controlled heating process which is based on parameters obtained purely by calculation, at a reduced heating output, is carried out, and the controlled heating process is stopped after a precalculated period of time.
Otherwise, i.e. if the temperature difference between the preselected target temperature and the measured starting temperature is greater than the minimum acceptable temperature differential, a regulated heating process is carried out. This means that the temperature is picked up at the temperature sensor and is compared with the preselected target temperature. In this process, the inertia of the temperature sensor and the heat capacity of the system are taken into account in the calculations. When a final temperature, which is below the preset target temperature of the system, is reached at the temperature sensor, the heating process is ended.
As a result of the above, after temperature equalization in the system, the target temperature is achieved with accuracy in a predefined tolerance band, without overshoot. Consequently, the time required for preparing the water is clearly shortened. Furthermore, heating output and thus electrical energy is saved, which would otherwise unnecessarily be used in the heating process. By dividing the method into two alternative implementation procedures, namely one procedure with a small gap between the target temperature and the starting temperature, and one procedure with a bigger gap between the target temperature and the starting temperature, safe functioning even in boundary states is ensured.
In this aspect, a starting temperature is sensed by the temperature sensor, then a starting temperature differential between the sensed starting temperature and a preselected target temperature is determined. If the starting temperature differential is less than a reference temperature differential, then the kettle is heated at less than full power. And, if the starting temperature differential is greater than the reference temperature differential, then an end temperature is determined and the kettle is heated until the end temperature is sensed at the temperature sensor.
In another embodiment, the heat capacity of the system is determined. The heat capacity may be a function of a heating output in the form of electrical power supplied, a temperature difference, and a period of time. According to another embodiment, in order to determine the heat capacity, the heating element is subjected to heating output only for a short time, so that even in the case of the fill quantity being minimal, an end temperature is not yet reached. After a brief switch-on phase, the heating element is switched off for a predefined waiting time. The waiting time is selected such that the thermal equalization processes in the liquid have been completed.
Another embodiment describes a possible definition for the reference temperature differential. The reference temperature differential is a function of the heat capacity, an electrical heating output of the heating element, and a delay time. The delay time is the amount of time, after activating the heating element, that passes before a temperature increase is sensed at the temperature sensor. In another embodiment, the reference temperature differential is the product of the electrical heating output of the heating element, and the delay time, divided by the heat capacity. Alternatively, fixed limiting values for the reference temperature differential can also be specified.
Another embodiment takes into account thermal equalization processes in the system and the inertia of the temperature sensor. Thus, the end temperature is the final temperature measured at the temperature sensor, at which final temperature the heating process is ended by the electronic regulator. In another embodiment, the end temperature is less than the target temperature.
In another embodiment, a response triggering temperature is dependent on the sensitivity of the temperature sensor. In this context, the response triggering temperature refers to the first measurable temperature that measurably differs from the starting temperature. The response triggering temperature differential is the difference between the starting temperature and the response triggering temperature.
In another embodiment, it is possible for the gradient of the temperature curve to be interpolated in a linear way over time by dividing the start temperature differential into small increments.
In another embodiment, regulated heating is carried out when the starting temperature differential equals the reference temperature differential. In another embodiment, the regulated heating is carried out by means of electronic regulating. In one embodiment, the kettle is heated at less than full power for a calculated period of time. In another embodiment, the calculated period of time for which the kettle is heated is such that, upon a first measurable temperature increase, the temperature measured at the sensor is less than the end temperature. In another embodiment, the kettle is heated at less than full power by intermittently activating and deactivating the heating element. In yet another embodiment, the kettle is heated at full power when the heating element is activated and the kettle is not heated when the heating element is deactivated. In one embodiment, the heating element comprises multiple heating units and at least one of the heating units is deactivated when the kettle is heated at less than full power.
In another embodiment, the end temperature designates the actual temperature of the system that occurs in a predefined tolerance band around the target temperature.
In one embodiment, the kettle is constructed such that the delay time is about equal to the time difference between the liquid reaching the end temperature and the end temperature being sensed at the sensor.
In one embodiment, a sampling temperature differential is determined over a sampling period of time, which begins after the delay time. In another embodiment, a sampling period of time is variable. In this way the sampling rate is determined. In another embodiment, a temperature gradient is determined as a function of the sampling time period and the sampling temperature difference. In another embodiment, the end temperature is extrapolated from a temperature curve based on the sampling time and sampling temperature differential. In another embodiment, several gradient values are weighted and averaged.
Finally, in another embodiment, the end temperature is situated in a predetermined tolerance range around the target temperature. Furthermore, the end temperature, at which the heating process is ended by the electronic regulator, is determined. Extrapolation can, for example, take place in a linear way.
In one aspect, an electric kettle for heating a liquid includes a container defining a cavity for containing the liquid, a heating element that transmits heat to the liquid, a temperature sensor responsive to a kettle temperature, a time sensor, and a heating regulator. The heating regulator is configured to, upon activation of the kettle, sense a starting temperature measured by the temperature sensor, determine a starting temperature differential between the measured starting temperature and a preselected target temperature, heat the kettle at less than a full power level for a calculated period of time in response to the starting temperature differential being equal to a reference temperature differential, and determine an end temperature and heat the kettle until the determined end temperature is measured at the temperature sensor in response to the starting temperature differential being greater than the reference temperature differential. In one embodiment, the heating regulator is also configured to determine a heat capacity. In another embodiment, the heating regulator is further configured to determine a delay time. In yet another embodiment, the heating regulator includes multiple heating elements.
Another aspect makes it possible to detect and react to a malfunction of the heating element or regulator. By the incorporation of system knowledge, it is possible to detect in a targeted way any errors and failures in the overall system of the regulator and/or heating element. In this way a situation can be prevented where during a malfunction, temperatures in the device are reached where only fusible cutouts or similar can still respond.
In another embodiment, as a result of the adaptability of the regulator, or of characteristic data stored in a storage element respectively, changes in the system, which correspond to expectations, can be taken into account. This relates in particular to different ambient temperatures that influence the initial temperature of the liquid to be heated, to different air pressures that influences the boiling temperature, and the like. In this way, predetermined disturbance variables can be taken into account.
In a further advantageous embodiment of the invention, the influence of disturbance variables, which cannot be exactly quantified, are taken into account. Accordingly, continuous adaptation that lasts over the entire lifetime of the device to changes and wear that cannot be predetermined, such as calcium buildup, becomes possible.
In still another advantageous embodiment, any malfunction of the device that cannot be rectified by self-adjustment of the regulator, or of the respective regulating variables, is signaled to the user. In this arrangement, for example light-emitting diodes, beepers/buzzers, or similar can be used as signal generators.
In another embodiment, by additionally recording a fill level of the water to be heated in the electric kettle, the heating time can be checked by computation. Detection of the fill level can for example take place by simple floats or by more sophisticated sensor equipment. However, it is also possible, by measuring the heating speed of the system per unit of time, to determine the quantity of water in the electric kettle, and to provide this value to the electronic system for further processing.
In another aspect, detection of, and reaction to, any malfunction of the heating element or of the electronic control becomes possible easily and safely.
In another embodiment, blinking light-emitting diodes or a loud beeper or buzzer alarm, for example, draw the user""s attention to the need for the device to be checked by a service technician.
In another embodiment, measuring the liquid level can, for example, take place mechanically by means of a float, or, as already described above. The measuring signal acquired in this way is then, for example, changed into an electrical signal by a potentiometer. After analog to digital conversion of the signal, the signal can be stored in the electronic memory.
In another embodiment, an additional sensor arrangement for measuring the ambient air pressure is provided in order to determine the respective boiling temperature and the boiling point in time at which boiling temperature is reached. A warm-up time for measuring an initial rise in temperature can also be matched to individual circumstances.
In another embodiment, by accommodating additional characteristic data it becomes possible to achieve smaller tolerances in calculated set point values. This contributes to lower energy consumption and, thus, to a more economical operation. Furthermore, for example, by a corresponding sensor arrangement, the water hardness can be determined, thus making it possible to draw conclusions concerning the calcium buildup in the device.
Finally, another embodiment prevents continued operation, over an extended period, of an electric kettle that is afflicted with serious malfunctions. In particular, non-approved manipulation of the electric kettle that, for example, serves the purpose of continuing to operate a defective electric kettle, can be prevented. Such device protection prevents the electric kettle from being switched on if a corresponding fault occurs particularly frequently. The device protection only permits continued operation if, during repair work, the error memory is reset by a service technician.
In one aspect, a method of detecting a malfunction in an electric kettle for heating a liquid includes sensing a starting temperature at the temperature sensor, activating the heating element for a selected length of time, sensing a second temperature at the temperature sensor at the end of the selected length of time, calculating a temperature differential as a difference between the starting temperature and the second temperature, and, in response to the calculated temperature differential being less than or equal to a reference temperature differential, deactivating the heating element.
In one embodiment, the method further comprises sensing a third temperature at the temperature sensor at a reference boiling time in response to the calculated temperature differential being greater than the reference temperature differential, deactivating the heating element in response to the third temperature being less than a reference boiling temperature, and deactivating the heating element in response to the third temperature being greater than or equal to the reference boiling temperature.
In a further embodiment the method includes determining the selected length of time, the reference temperature differential, the reference boiling time, and the reference boiling temperature as functions of an ambient temperature. In another embodiment, determining the selected length of time, the reference temperature differential, the reference boiling time, and the reference boiling temperature comprises accessing data of a characteristic data matrix stored in the electronic memory.
In another embodiment, the method includes indicating a malfunction to a user if the calculated temperature increase is less than or equal to the reference temperature increase and if the third temperature is less than the reference boiling temperature. In another embodiment, indicating the malfunction comprises activating an acoustic indicator.
In one embodiment, the method includes measuring a liquid fill level prior to activating the heating element. In another embodiment, the selected length of time, the reference temperature differential, the reference boiling time, and the reference boiling temperature are functions of the liquid fill level.
One embodiment includes modifying the data of the characteristic data matrix in response to the third temperature being greater than or equal to the reference boiling temperature. Another embodiment includes determining a power consumption of the heating element over time and a temperature gradient of the liquid over time.
In another embodiment, the method includes storing system errors, wherein, in response to the system errors occurring above a predetermined acceptable frequency, the heating element is deactivated until a memory is reset. In another embodiment, the system errors include calculating the temperature differential to be less than or equal to the reference temperature differential and sensing the third temperature to be less than the reference boiling temperature.
In another aspect, an electric kettle for heating a liquid includes a heating element that transmits heat to the liquid, a temperature sensor responsive to a kettle temperature, a time sensor, an electronic memory that stores characteristic data, and a heating regulator in communication with the memory.
In one embodiment, the heating regulator is configured to sense a starting temperature measured by the temperature sensor, activate the heating element for a selected length of time, sense a second temperature measured by the temperature sensor at the end of the selected length of time, calculate a temperature differential as a difference between the starting temperature and the second temperature, and deactivate the heating element in response to the calculated temperature differential being less than or equal to the reference temperature differential.
In another embodiment, heating regulator is further configured to sense a third temperature measured by the temperature sensor at a reference boiling time in response to the calculated temperature differential being greater than the reference temperature differential, deactivate the heating element in response to the third temperature being less than a reference boiling temperature, and deactivate the heating element in response to the third temperature being greater than or equal to the reference boiling temperature.
In another embodiment, the memory comprises a characteristic data matrix including data corresponding to the selected length of time data, the reference temperature differential, the reference boiling time, and the reference boiling temperature. In a further embodiment, the kettle further includes a microprocessor that updates the characteristic data in response to a system change. According to another embodiment, the system change is a decrease in heating output of the heating element.
In another embodiment, the electric kettle further includes an indicator to indicate a malfunction to a user. In a further embodiment, the indicator is an acoustic indicator.
In one embodiment, the electric kettle includes a liquid level sensor that measures a level of the liquid in the kettle.
In a further embodiment, the heating regulator is further configured to deactivate the heating element until the memory is reset in response to system errors occurring above a predetermined acceptable frequency.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.